Search

Fe (111) single crystal films have been grown on GaAs (111) by the sputter-beam method. Reflection high energy electron diffraction and x-ray diffraction measurements have revealed excellent aligned epitaxial growth of the Fe (111) film on GaAs. No evidence of preferential diffusion of As atoms into the deposited Fe layers has been found in the depth profile of Auger electron spectra. Magnetic measurements clearly show that the Fe (111) film has nearly ideal threefold symmetric anisotropy and exhibits isotropic initial permeability in the film plane, as predicted by theory.

The relationship between magnetic softness and the precipitation process of highly resistive soft magnetic TM-M-X [TM: Fe, Co, M: Metal, X: O, N] films was investigated.The as-sputtered and incompletely phase-separated state of Fe-Al-O, Fe-Sm-O, and Fe-V-O films show magnetic softness. This soft magnetic property is attributed to the exchange coupling of neighboring magnetic crystals through the intergranular phase. However, Fe-B-N and Co-Sm-O films show perpendicular anisotropy over the whole annealing temperature range, because it is easy to separate TM-X and M-X owing to their extremely high diffusion rate and the large difference between their Gibbs free energies (−ΔG°diff). These results indicate that the small diffusion coefficient of M and X in TM (D), the large critical radii of crystal nucleation of M-X compounds (r*), the high melting points of M-X compounds (Tm) and the large −ΔG°diff. are essential factors in the formation of stable hetero-granular structures with both magnetic softness and high resistivity.

The precipitation process of highly resistive soft magnetic Fe-M-X (M=Al, B, Sm, V; X=O, N) films was investigated. The process can be understood in terms of a difference of Gibbs free energies between M-X and Fe-X, the atomic diffusion rate for M, X in α-Fe, and crystallization of the intergranular phase. Fe-(Al, Sm)-O films, which have large -ΔG°diff, exhibited a stable fine crystal structure in the as-sputtered state. By analyzing the precipitation process, using Johnson-Mehl-Avrami plotting, we found that the slow atomic diffusion in α-Fe brings about the lowest coercivity. Specifically, Fe-(Al, Sm)-O films, which have low diffusion rates, have exhibited Hc of 0.5-1.0 Oe, while Fe-V-O films, which have also low diffusion rates, showed a rapid and distinct crystal growth of the intergranular phase (V-O) at an early stage of the precipitation process. This allowed a rapid growth of α-Fe, which had been suppressed in the other Fe-M-X systems.

The first and second order energy terms of uniaxial magnetic anisotropy, K/sub u1/ and K/sub u2/, of CoPtCr-SiO/sub 2/ perpendicular recording media were studied as a function of film composition and film structure. Moreover, the fabrication of CoPtCr-SiO/sub 2/ media having adequate values of K/sub u1/ and K/sub u2/ for high recording density was preliminarily studied. An analysis of CoPtCr films with various kinds of seed layer materials revealed that the values of K/sub u1/ and K/sub u2/ varied significantly with the seed layer material used, probably due to epitaxial growth of CoPtCr on the seed layers. K/sub u2/ increased, accompanied by a reduction in K/sub u1/ as the c/a ratio of the hcp-CoPtCr lattice increased. Moreover, experimental results suggest that a high stacking-fault density enhances the K/sub u2/ values, especially in the high Pt content region. The addition of SiO/sub 2/ reduces the total anisotropy, K/sub u1/+K/sub u2/, but no significant change in the ratio of K/sub u2//K/sub u1/ was observed. The fabrication of CoPtCr-SiO/sub 2/ media with adequate values of K/sub u1/ and K/sub u2/ was successfully demonstrated, although more intensive efforts to enhance grain isolation are required to confirm the advantages conferred by the K/sub u2/ term.

In this study, magnetic softness of permalloy granular films produced by co-evaporation is investigated. Py (Ni/sub 80/Fe/sub 20/) is used as ferromagnetic source and one of MgF/sub 2/, SrF/sub 2/, and CaF/sub 2/ as matrix source. Resistivity is found to be about the same for all matrices and it extends to 10/sup 5/ /spl mu//spl Omega/cm. Above this limit, the films tend to exhibit superparamagnetic behavior locally and a hard-to-saturate curve is superimposed in their magnetization curves. As far as the ferromagnetic metal is Py, the anisotropy field Hk is about 50 Oe and this is common for all matrices. Perpendicular anisotropy is exhibited by a few series of samples such as Py-CaF/sub 2/ and Py-MgF/sub 2/. Therefore, by co-evaporation, magnetic softness, high resistivity and well defined frequency profiles are available in the as-made state for a variety of matrices with Py as a ferromagnetic component.

Phase separation behavior of Co-Mo thin films and magnetic anisotropy energy of Co-Mo and Co-Mo-Cr thin films deposited on Cr(112)//MgO(110) single crystal substrates have been studied by using an analytic transmission electron microscope and a vibrating sample magnetometer. The results show that the phase separation does happen in the Co-Mo thin films, resulting in Mo-rich phase surrounding Co-rich magnetic grains, similar as that in the Co-Cr system. However, the Mo-rich phase still shows the ferromagnetic characteristic at room temperature due to its not so high Mo content. The first order anisotropy constants of the Co-Mo thin films are about double that of the Co-Cr thin films in the composition range of 4.0-14.0 at.%Mo (or Cr). With the addition of Cr into the Co-8.7 at.%Mo thin film, the magnetic anisotropy constants show no obvious deterioration, but it results in the increase of anisotropy field.

We have studied the exchange coupling of antiferromagnetic NiO/ferromagnetic Ni 80 Fe 20 bilayers, and have found the coupling strength is closely related to the crystallographic orientation of the NiO at the NiO/Ni 80 Fe 20 interface. As the (111) of NiO, which is a parallel spin plane, becomes predominant at the interface, the coupling is enhanced. For the development of a spin-valve sandwich utilizing NiO as an antiferromagnetic (AF) layer, the configuration S/AF/F/NF/F (S = substrate, F = ferromagnetic layer, NF = nonmagnetic spacer) is more favorable for large magnetoresistance than the S/F/NF/F/AF configuration, because the former gives a high exchange field and a smooth NF spacer, resulting in an ideal antiparallel spin alignment in the sandwich during the magnetization process. Moreover, we have obtained a magnetoresistance ratio of 8% for Ni 80 Fe 20 and 15% for Co at the optimum ferromagnetic layer thickness of about 50 A.

We have prepared epitaxial Fe‐N martensite (α′) films on α‐Fe (001) single‐crystal underlayers by the sputter beam method and synthesized a metastable α″‐Fe16N2 phase by post‐annealing. The saturation magnetization (4πMs) of the α′ phase films is in the range of 21.9–23.3 kGauss which is larger than that of α‐Fe. Magnetization of the annealed samples containing α″‐Fe16N2 phase have increased with the x‐ray diffraction intensity ratio of α″(002) to [α″(004)+α′(002)]. Very careful analysis of x‐ray diffraction data gives more reliable value on the volume fraction of the α″ phase in the annealed samples. As a result, it is found that the Fe‐N film with magnetization of 24.7 kG contains 32 vol %‐α″ phase within it. This result leads us to believe that α″‐Fe16N2 phase has a remarkably large saturation magnetization.

The dynamic behavior of the magnetization for (111)‐oriented films with cubic crystalline structures has been studied theoretically, and the permeability for such textured films has been confirmed to be isotropic. Meanwhile, Co‐rich CoFeAlCu films with high saturation magnetization (4πMs=15.8 kG) and low magnetostriction (λs=1.3×10−6), prepared by the dc sputtering method, are found to exhibit perfect fcc (111) orientation with grain size of about 400 A. A fairly good isotropic magnetic softness has been attained for CoFeAlCu films with Zr underlayers even in the as‐sputtered state with Hc of about 1.5 Oe in every measuring direction. After annealing in a rotating magnetic field at 380 °C for 2 h, the magnetic softness is further improved with Hc as low as 0.85 Oe, and the permeability is isotropic in the film plane and as high as 500–600. The perfect (111) orientation of the CoFeAlCu films leads to the isotropic softness, which is consistent with the theoretical prediction.

Crystallographic texture of polycrystalline Co films prepared by sputtering on a variety of underlayers has been investigated. The texture of polycrystalline Co films greatly depends on underlayer material, and this dependence is well explained by taking into account the surface free energy difference between the underlayer material (γu) and Co (γCo). When γu is larger than γCo, Co films can be grown with a well‐defined crystallographic texture of hcp‐Co(001), which is the lowest surface energy crystal plane in order to minimize the total surface energy. In contrast, if γu is much smaller than γCo, no remarkable underlayer effects can be observed in the crystallographic structure of Co films.

Microwave-assisted magnetization switching (MAS) in a granular magnetic film is examined by computer simulation. Contrary to the macrospin calculation and the experiments on single magnetic dots reported so far, in which the switching field linearly decreases with increasing rf frequency and then sharply increases at the critical frequency, the granular film exhibits considerably broad MAS behavior against the rf frequency. This broad MAS behavior is mainly caused by the dispersion of magnetic properties and thermal agitation. On the other hand, intergrain dipolar and exchange interactions enhance the MAS effect in the granular film and suppress the MAS broadening.

To realize a granular film composed of L10-FePt grains with high uniaxial magnetic anisotropy energy, Ku, and segregants for energy-assisted magnetic recording, a FePt-GeO2/FePt-C stacked film was investigated in the engineering process. The FePt-GeO2/FePt-C stacked film fabricated at a substrate temperature of 450 °C realized uniaxial magnetic anisotropy, Kugrain, of about 2.5 × 107 erg/cm3, which is normalized by the volume fraction of FePt grains, and a granular structure with an averaged grain size of 7.7 nm. As the thickness of the FePt-GeO2 upper layer was increased to 9 nm, the Ku values were almost constant. That result differs absolutely from the thickness dependences of the other oxide segregant materials such as SiO2 and TiO2. Such differences on the oxide segregant are attributed to their chemical bond. The strong covalent bond of GeO2 is expected to result in high Ku of the FePt-GeO2/FePt-C stacked films.

Phase separation of Co-W sputtered thin films having a large magnetocrystalline anisotropy energy have been investigated. A nanoscale compositional fluctuation caused by magnetically induced phase separation was directly confirmed in the films deposited on a heated substrate in analogy with Co-Cr-based alloys. The difference between the phase separation features in Co-W and Co-Cr is attributed to the difference in their elastic energy. It is expected that the phase separation is enhanced by selecting optimum sputtering conditions. The Co-W system, therefore, is considered to be a promising candidate as a base alloy system for high-density recording media.

The perpendicular magnetic anisotropy K/sub u/ of CoPtCr films deposited on Ru seed layers is discussed as a function of film composition. Moreover, the change in K/sub u/ by the addition of SiO/sub 2/ is examined in light of thermal stability of CoPtCr-SiO/sub 2/ media. The K/sub u/ of Co-10 at % Cr shows a high value of /spl sim/5/spl times/10/sup 6/ erg/cm/sup 3/, which is much higher than that reported by Bolzoni et al. The addition of Pt to the Co-10 at % Cr film results in a further enhancement of K/sub u/. A maximum K/sub u/ of nearly 10/spl times/10/sup 6/ erg/cm/sup 3/ is observed at 25/spl sim/30 at % Pt. All series of films with various Cr contents show maximum values of K/sub u/ at 25/spl sim/30 at % Pt. The maximum value for CoPt (Cr=0) films reaches /spl sim/15/spl times/10/sup 6/ erg/cm/sup 3/. The value of magnetocrystalline anisotropy of grains K/sub u//sup g/, calculated by taking account of the volume fraction of CoPtCr grains, decreases as the SiO/sub 2/ content increases. However, the K/sub u//sup g/ maintains a large value of more than 7/spl times/10/sup 6/ erg/cm/sup 3/, even at 10 at % SiO/sub 2/ in addition to (Co/sub 90/Cr/sub 10/)/sub 80/Pt/sub 20/, which indicates a high potential of CoPtCr-SiO/sub 2/ media to resist thermal agitation of magnetization.

The addition of Ru to L10 (fct)-FePt thin films was investigated to lower the Curie temperature (T c) of the films, which is beneficial for heat-assisted magnetic recording. The FePtRu films exhibited L10-ordering and perpendicular anisotropy for Ru contents up to at least 18 at. %, and T c was reduced by ∼100 °C/6 at. % Ru independent of the Fe/Pt ratio between 0.8 and 1.1. The T c versus uniaxial magnetic anisotropy diagrams exhibited a linear trend similar to that of FePtCu films but with a larger slope. Systematic analysis clarified that the Ru additive caused a slight decrease of the anisotropic field but greatly decreased the saturation magnetization, which was related to the observed reduction in T c.

The crystalline structures and magnetic properties of Co-Fe and Co-Fe-Al films with various buffer layers (Cu, Permalloy and Zr), grown on glass or Si substrates, have been studied. It was found that the Co-Fe films on Zr underlayers exhibit very good FCC (111) orientation parallel to the film plane, whereas neither Cu nor Permalloy can suppress the intrusion of HCP phase in the Co-Fe films. Furthermore, the FCC (111) orientation is greatly enhanced by adding a small amount of Al to the Co-Fe films with Zr underlayers. The magnetic softness expected for the films with (111) orientation of the FCC phase was not attained for the Co-Fe films because of residual HCP phase and deterioration of the (111) orientation by annealing. On the other hand, the Co-Fe-Al films with Zr buffer layers, which have a single FCC phase with enhanced (111) orientation, are magnetically soft.

We have studied the magnetization switching behavior in single nanodots of an epitaxially grown Co/Pt multilayer with various dot diameters (d) ranging from 120 to 1600 nm. The switching behaviors in all the dots used in this study are considered to be of the nucleation type from the angular dependence of the switching field. However, detailed analysis of the thermally activated magnetization switching reveals that there is a critical diameter (d c) of 400 nm above which the switching process changes. For d < d c, the switching behavior of the dot is well explained by the nucleation process based on the single-energy barrier model. On the other hand, the switching behavior for d ≥ d c obviously deviates from that for d < d c and can be plausibly explained by the multi-barrier model.

Ultra high-density-recording states were observed in magneto-optical (MO) and magnetic recording media, using particles prepared by sputtering. We clearly observed recording bits as small as 0.3μm in diameter for MO media,and also visualized the influence of the recording laser power on high-density recording states. The observation method also made it possible to observe ultra-high-density recording states (with a bit length of 0.08μm) in obliquely evaporated Co-CoO magnetic recording media. According to these experimental results, the resolution of the present method was estimated to be finer than 0.08μm.

Based on the Stoner-Wohlfarth model, we have studied the effects of higher order magnetic anisotropy terms on the dynamic coercivity of perpendicular magnetic recording media. The waiting time dependence of the dynamic coercivity Hc becomes very gradual due to the presence of the second-order anisotropy constant, reflecting the fact that the higher-order anisotropy appreciably enhances the energy barrier for thermal stability. It is found that the behavior of Hc can be perfectly described by modifying the well-known Sharrock formula as Hc\fallingdotseqHK {1-[(kBT/K*V)ln ( f0t'/ln 2)]n} and the higher-order anisotropy term appreciably enhances both the effective anisotropy constant K* and the index n.

Magnetically induced phase separation along the Curie temperature in the hexagonal-close-packed phase of a Co–Mo binary system has been predicted by thermodynamic calculations. Furthermore, the phase separation and magnetic properties of Co–Mo sputtered thin films have been investigated. Nanoscale compositional fluctuation caused by the magnetically induced phase separation has been confirmed in the films deposited on a heated substrate in the same manner as Co–Cr-based alloys. The magnetic anisotropy constant of the Co–Mo films is larger than that of Co–Cr films. From these results, it is concluded that the Co–Mo system is promising for use as a base alloy system for high density recording media.

Magnetization of an isolated single dot as small as 60 nm in diameter fabricated from a single crystal L10-FePt(001) film has been measured by detection of the anomalous Hall effect in the temperature range from 10 to 300 K. Over the whole temperature range, the dots with diameter ranging from 60 nm to 12 μm exhibit perfect rectangular magnetization loops with coercivity almost constant regardless of the very large difference in diameter. The activation energy has been evaluated to be about 4×10−19 J, equivalent to the domain-wall energy times the square of the domain-wall thickness, suggesting that the magnetization reversals are initiated by nucleation of reversed embryo with the dimension of the exchange length.

We searched for an optimum thermal treatment of double or quadruple layers of (Fe 2.5 nm/Pt 2.5 nm) in ordered to obtain a monolayer assembly of ordered FePt particles. It was found that by controlling the thickness, the substrate temperature, and using MgO(100) substrates, monolayer assemblies of ordered FePt particles with the crystal axes [001] aligned vertically can be obtained at relatively low substrate temperatures around 400 °C.

We have studied the effects of higher order magnetic anisotropy terms on the irreversible switching field, the energy barrier, and the dynamics of magnetization switching of an uniaxial fine particle. The higher order terms effectively enhance the energy barrier without notable change in the switching field, enabling us to attain both saturation recording and good thermal stability. It is found that dynamics of the magnetization vector slightly slows down due to the presence of the higher order terms, but the effect is very small when they are not so large with respect to the lowest order anisotropy term.

We have studied the switching behavior of a single Co/Pt multilayer dot under the assistance of rf fields. The switching field monotonically decreases with increasing rf frequency up to a critical frequency. It is found that the reduction of the switching field is more significant than the theoretical prediction based on the single macrospin model. In addition, switching field distribution due to thermal fluctuation is also considerably suppressed. The simulation has revealed that these drastic changes are caused by excitation of large amplitude spin waves in the dot.

Recording states of obliquely evaporated Co‐O thin film media with different noise levels are observed by using ultrafine Co particles fabricated by a sputtering method. Very clear bit patterns with high resolution and high contrast are obtained by the method. This method enables us to estimate the fluctuation of magnetization transitions in the thin film media. It is also experimentally confirmed that the modulation noise level increases as fluctuation in the magnetization transitions become great.

High density recording patterns were visualized by sputter-deposition of magnetic fine particles on recording media. Very clear patterns were formed when the magnetic particles were deposited under a weak magnetic field onto the sample surface covered with a thin oil layer. By using this process, the recorded patterns at a density of 100 k FCI (bit length 0.25 μm) were very clearly observed by a scanning electron microscope. This method gives very effective measures to investigate the recording states of magnetic and magneto-optical media.

書誌情報のみ, Bibliographic data Only

In order to clarify the relation between the magnetically induced phase separation and the recording media characteristics, the thermodynamic calculations of Co–Cr–Pt–B and Co–Cr–Pt–Ta systems have been carried out by using the available binary assessment data and Miedema’s semiempirical values. B is segregated to the boundary in a similar manner as Cr, which makes the boundary region paramagnetic. This result is consistent with available data that B weakens the intergranular magnetic coupling and increases the magnetic anisotropy in Co–Cr–Pt recording media. By adding Ta, the Cr content in the paramagnetic phase is also increased, reducing the intergranular magnetic coupling. However, the Ta content in the ferromagnetic phase is higher than in the paramagnetic phase, decreasing the magnetic anisotropy. Accordingly, the thermodynamic calculations successfully explain experimental magnetic data for Co–Cr–Pt–B and Co–Cr–Pt–Ta recording media.

Ordering process of FePt fine grains in SiO 2 matrix is investigated. Study of the grain growth process by annealing FePt–SiO 2 granular films with various packing densities monitoring the ordering parameter reveals that ordering is triggered and accelerated by coalescence of grains.

Magnetic and structural properties of Pd/Co/Pd trilayers in hydrogen atmosphere (pressure P H ) have been investigated. The Pd layers are well hydrogenated and fully transformed to β-Pd–H phase at P H =1 atm, but this β phase is distorted along the film normal under the restriction of substrate, and the atomic ratio of H/Pd is estimated to be 0.6. Hydrogenation at low P H gives rise to enhancement of perpendicular magnetic anisotropy K eff . For instance, 3 A-thick Co at P H =1.3 atm exhibits very high K eff (3.5×10 7 erg/cm 3 ) being two times larger than that of as-prepared one (1.6×10 7 erg/cm 3 ). Further increase in P H decreases K eff gradually, while K eff recovers to some extent after full dehydrogenation.

The origin of the large perpendicular magnetic anisotropy (0.5–1×10−6 erg/cm3) of Ni‐polyethelene coevaporated films has been studied. It is indicated that the anisotropy can be very well explained by uniaxial anisotropy induced by the large tensile stress (1–3×1010 dyn/cm2) through the inverse magnetostrictive effect. Other factors, such as magnetocrystalline or shape anisotropy, produce little effect on the perpendicular anisotropy of the films.

We have investigated the effects of B and C on the ordering of L10-CoPt films and confirmed the following quite different role between B and C in CoPt. A small amount of B considerably decreases the temperature for ordering and is interstitially incorporated into L10-CoPt along its c axis due to the large negative heat of solutions with Co and Pt. X-ray photoelectron spectroscopy analyses have revealed that B atoms combine with Co and Pt. In contrast, C is not incorporated into the CoPt lattice due to its positive heat of solution, resulting in no decrease in the ordering temperature. The present results reveal that CoPt films containing a small amount of B are promising for high density recording media.

For a circular dot with the lateral size larger than the exchange length l/sub ex/(=A/sup 1/2//M), it is well known that the stable remanent state is a vortex structure where the spins around the center of the dot direct up- or down-ward. If only these two states are allowed in the vortex structure, it is possible to use it as a bi-stable memory element. In order to check this possibility, we have investigated vertical magnetization process of various sub-micron permalloy dots by PMOKE and MFM. Polycrystalline circular dots with 180-4000 nm in diameter and 40-80 nm in height were fabricated using electron-lithography and a lift-off technique. While PMOKE measurements show very low remanence against vertical field, MFM detects distinct spin vortex structures where spins around the dot centers direct up-or down-ward. This result suggests a possibility of spin vortex structures as bi-stable switching memory elements, although their averaged remanence is quite small. From LLG calculations, it is found that lateral dimension of the spin vortex coincides with the exchange length l/sub ex/=A/sup 1/2//M/sub s//spl sim/15 nm regardless of the dot diameter. Furthermore we have established a magnetic phase diagram of permalloy dots as functions of dot diameter and height.

We have explored magnetic properties and structural characteristics of Fe/Pt multilayers after annealing at various temperatures in order to clarify the growth process of the ordered Fe–Pt phase in the Fe/Pt multilayer structures. It is found that rapid diffusion at Fe/Pt interface occurs at temperatures 275–325 °C and the multilayer structure directly transforms to the ordered (fct) phase when Fe and Pt layer thickness is almost equal. The ordering parameter S is evaluated to be 0.50–0.65 after annealing at 300–325 °C, and then is significantly enhanced to ∼1.00 with increasing annealing temperatures. By comparing the thermal processes of these different multilayer structures, it is found that the rapid formation of the fct phase in the multilayers with Fe/Pt≅1 is due to relatively rapid diffusion at the interface.

We have studied the effect of additional elements of Sn, Pb, Sb, and Bi on the ordering of L10–CoPt films. All of these additives are demonstrated to be very effective to promote the ordering and developing of a very large coercivity of the samples annealed at 400 °C. It is worth noting that this annealing temperature for ordering is 200 °C lower than that of pure CoPt. The crystallographic and chemical analyses have revealed that these additives easily diffuse and segregate onto the film surfaces by postannealing because of their very low surface free energy and extremely low solubility in Co. Therefore, it seems reasonable to conclude that the ordering in the CoPt film is significantly promoted at much lower temperature by the aid of a lot of defects produced by the additives excreted by postannealing.

Thermodynamic calculations of the magnetically induced phase separation of the hcp phase in Co–Cr-based alloy systems, which is the origin of the compositional modulation observed in magnetic recording media, have been carried out. The magnetic and nonmagnetic terms in the Gibbs energy are evaluated from available thermodynamic data and Miedema’s semiempirical values. It is demonstrated that the phase separation in Co–Cr–X ternary alloys can be classified into four types, depending on the values of interaction energies between Co and/or Cr and X atoms. Existing magnetic data on Co–Cr-based recording media are discussed in terms of the phase stability of the hcp phases. The present calculations would be useful for development of high-density magnetic recording media.

The magnetization reversal processes in Fe/sub 20/Ni/sub 80/ dot arrays have been studied both experimentally and theoretically. The circular-shaped dots with thickness of 60 nm and variable diameters of 0.2-0.8 /spl mu/m were fabricated by using electron-beam lithography and lift-off technique. A size dependent vortex nucleation, annihilation and initial susceptibility were determined from magneto-optical hysteresis loops. With increasing dot diameter the nucleation and annihilation fields decrease, whereas the initial susceptibility exhibits a linear increase. The experimental data were compared to the results of analytical calculations taking into account magnetostatic interactions, exchange and Zeeman energies for a planar array of magnetically soft circular dots.

We have studied the magnetic properties of bcc-Fe fine particles with a diameter D of 3/spl sim/40 nm prepared by a vapor condensation process. The particles exhibit uniaxial anisotropy of 2.0/spl sim/3.0/spl times/10/sup 6/ erg/cc which is much larger than that of bulk Fe (0.5/spl times/10/sup 6/ erg/cc). Numerical calculations taking into account dipolar interaction and thermal agitation reproduce the particle size dependence of coercivity very well. According to the present work, magnetization reversal of Fe particles proceeds via coherent rotation for D 15/spl sim/20 nm. It is, however, still uncertain why the magnetic anisotropy of Fe fine particles is uniaxial and so large.

We have fabricated a magnetic rectangular block array of 130 Gblock/in.2 with a block-size-to-period ratio of 0.85 by using focused ion beam lithography. Each block has perpendicular crystal magnetic anisotropy and a single domain structure. Its advantage as patterned media is discussed in conjunction with thermal stability in recorded magnetization, reading signal, and area density.

Ni 80 Fe 20 permalloy nanoparticles (NPs) with narrow size distribution and homogeneous composition have been prepared by the polyol processing at 180 °C for 2h and their particle sizes can be tunable in the size range of 20∼440 nm by proper addition of K 2 PtCl 4 agent. X-ray diffraction results show that the NiFe NPs are of face centered cubic (FCC) structure. The addition of K 2 PtCl 4 does not affect the composition of NiFe NPs but decreases the particle size remarkably. Both saturation magnetization and coercivity of the as-prepared NiFe NPs decrease with decreasing particle size. Annealed at 280 °C, however, the saturation magnetization of various size NiFe nanoparticles increases drastically and approaches to the bulk for the ∼440nm NiFe particles, however, a maximum coercivity (∼270 Oe) happens at a critical size of ∼50 nm. The magnetic property dependency of these NiFe NPs on annealing has been discussed by considering the surface chemistry.

We propose a simple method to evaluate the first and second magnetic anisotropy constants of high-density magnetic recording media with their easy axis in the film plane. By measuring the magnetization curve along the film normal, uniaxial anisotropy constants K1 and K2 can be determined simultaneously. The present method has been applied to (11⋅0) oriented CoCrPtTa hard disk media. The measured anisotropy constants are presented along with their temperature dependence. We also point out serious shortcomings of the currently used evaluation methods, such as the rotational hysteresis method and the singular point detection technique, which usually overestimate the magnetic anisotropy constant.

A stress-operated memory device consisting of an ellipsoidal magnetic particle array and an electrostrictive grid is proposed. In the device, the magnetic state of the particle can be controlled only by the magnetostriction effect. Each particle is located at the intersection of the grid and has an in-plane uniaxial anisotropy. A pair of electric contacts is connected to the end of each wire. In the writing process, the driving voltages are simultaneously applied to two pairs of the selected contacts. This allows to apply a local electric field whose direction and amplitude can be regulated by varying the voltage intensity and polarity. The exerting stress on the magnetic particle results in the linear magnetostriction and hence an additional anisotropy energy in the particle. The in-plane total energy minimum, corresponding to the magnetization direction, follows the local electric field. Consequently the magnetization of the single magnetic particle located at the intersection can therefore be selective...

The structure and magnetic properties of Co1−xPtx/SiO2 granular films with the Pt atomic content x varying from 0.42 to 0.71 have been investigated. It was found that the CoPt grains transform from disordered fcc to the ideal ordered L10 structure after annealing at 680 °C. In the range of x=0.42–0.60, the lattice parameter ratio a/c slightly varies from 0.965 to 0.979. For the Pt content x∼0.7, the superstructure exists but the lattice constant ratio is equal to 0.998. The coercivity dramatically increases from 1.8 to 9.1 kOe and then decreases to several hundred Oe while x increases from 0.42 to 0.71 for the grain size d∼10 nm. We also found that the coercivity increases with the grain size. Our results show that the highly ordered fct structure, accompanied by the high crystalline anisotropy, is a prerequisite for the high coercivity, and the coercivity can be tailored by controlling either the composition or the grain size.

Fe/Fe1−xSix multilayers show distinct antiferromagnetic (AF) coupling for a wide spacer composition range 0.50 0.70. The quantity Δ was evaluated both from I–V characteristics and the temperature dependence of the resistivity with the current perpendicular to the sample plane using a crossed electrode geometry junction. It is found that the barrier height increases from 0.15 to 0.70 eV with increasing the Si content x. These values almost agree with the para...

The magnetization reversal of a nanometer size ferromagnetic particle with surface anisotropy is investigated by micromagnetic computer simulation. Two kinds of calculations were performed: calculations with and without thermal fluctuation. In the case without thermal fluctuation a surface anisotropy of several ergs/cm/sup 2/ was found to increase the magnetization reversal field of the particle by a factor of ten as compared with the case without surface anisotropy. The introduction of surface anisotropy was also found to increase the reversal field, and hence, improve thermal stability.

We have demonstrated a thermomagnetic writing on a 29 Gbit/in.2 patterned medium with perpendicular magnetic anisotropy, by using Joule heat produced by a current flowing between a magnetic dot in the medium and a magnetic force microscope tip. The possible application of this thermomagnetic method to future patterned media is also discussed.

Magnetization reversal processes of submicron NiO/Fe19Ni81 exchange-coupled Hall cross with a variable width in the range from 0.2 to 0.4 μm were studied by the magnetoresistivity and the planar Hall effect measurements. The magnetization reversal was found to take place via a coherent rotation in the Hall cross, suggesting that the size of the antiferromagnetic domain is regulated by the wire width. The magnitude of the exchange coupling field Hex varied in proportion to the inverse wire width.

Magnetization reversal processes of the Fe 19 Ni 81 /NiO exchange-coupled bilayers cut into submicron scale wires with a variable width in the range from 0.5 to 10 μm were studied by analyzing the anisotropy magnetoresistance and the planar Hall effect. The demagnetization process determined form the anisotropy magnetoresistance exhibited the stepwise reversal, indicating the presence of oppositely exchange coupled regions. The size effect on the exchange coupling field was observed when the wire is finer than 1 μm. The large planar Hall effect of 25% was found to occur at room temperature in the Hall cross 0.5 μm in width due to the nucleation of the inclined domain wall.

Arrays of submicron polycrystalline and epitaxially grown (1 0 0) hcp Co dots and antidots were prepared in order to study the magnetization reversal processes influenced by well defined anisotropy, dipolar and exchange fields. The magnetization reversal processes for both polycrystalline and epitaxial Co dot arrays are found to be initiated by domain wall nucleation. The domain wall propagation is regulated by the evolution of the effective internal field. The antidots exhibit perculiar stripe domains in the remanent state. the direction of the stripes is well determined by the applied magnetic field direction.